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   难度：Medium
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  <div>
   <h1 class="question_title">
    216. Combination Sum III
   </h1>
   <div>
    <p>
     Find all possible combinations of
     <i>
      <b>
       k
      </b>
     </i>
     numbers that add up to a number
     <i>
      <b>
       n
      </b>
     </i>
     , given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
    </p>
    <p>
     <strong>
      Note:
     </strong>
    </p>
    <ul>
     <li>
      All numbers will be positive integers.
     </li>
     <li>
      The solution set must not contain duplicate combinations.
     </li>
    </ul>
    <p>
     <strong>
      Example 1:
     </strong>
    </p>
    <pre>
<strong>Input:</strong> <i><b>k</b></i> = 3, <i><b>n</b></i> = 7
<strong>Output:</strong> [[1,2,4]]
</pre>
    <p>
     <strong>
      Example 2:
     </strong>
    </p>
    <pre>
<strong>Input:</strong> <i><b>k</b></i> = 3, <i><b>n</b></i> = 9
<strong>Output:</strong> [[1,2,6], [1,3,5], [2,3,4]]
</pre>
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   <h1 class="question_title">
    216. 组合总和 III
   </h1>
   <p>
    找出所有相加之和为&nbsp;
    <em>
     <strong>
      n
     </strong>
    </em>
    的&nbsp;
    <strong>
     <em>
      k&nbsp;
     </em>
    </strong>
    个数的组合
    <strong>
     <em>
      。
     </em>
    </strong>
    组合中只允许含有 1 -&nbsp;9 的正整数，并且每种组合中不存在重复的数字。
   </p>
   <p>
    <strong>
     说明：
    </strong>
   </p>
   <ul>
    <li>
     所有数字都是正整数。
    </li>
    <li>
     解集不能包含重复的组合。&nbsp;
    </li>
   </ul>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <pre><strong>输入:</strong> <em><strong>k</strong></em> = 3, <em><strong>n</strong></em> = 7
<strong>输出:</strong> [[1,2,4]]
</pre>
   <p>
    <strong>
     示例 2:
    </strong>
   </p>
   <pre><strong>输入:</strong> <em><strong>k</strong></em> = 3, <em><strong>n</strong></em> = 9
<strong>输出:</strong> [[1,2,6], [1,3,5], [2,3,4]]
</pre>
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